WiP: Support for MVAPICH3

[sheepforce]

Adds another cabal flag for MVAPICH3 support. In MVAPICH3 the pkg-config name was changed from mvapich2 to merely mvapich, so not both versions can simultaenously be handled by the same flag.

TODO:

• [ ] Wait for https://github.com/NixOS/nixpkgs/pull/293499 to be merged and update the Nix flake to have MVAPICH3 in the Nix build